Surface acoustic wave (SAW) devices use one or more interdigitated transducers (IDTs), and perhaps reflectors, provided on a piezoelectric substrate to convert acoustic waves to electrical signals and vice versa. SAW devices are often used in filtering applications for high-frequency signals. Of particular benefit is the ability to create low loss high order bandpass and notch filters without employing complex electrical filter circuits, which may require numerous active and passive components. A common location for a filtering application is in the transceiver circuitry of wireless communication devices.
With reference to FIG. 1, a typical SAW device 10 on a temperature compensated bonded substrate is illustrated. The SAW device 10 will generally only include a piezoelectric substrate 12, which has a surface on which various types of SAW elements, such as IDTs and reflectors, may be formed. In a temperature compensated bonded substrate, the piezoelectric substrate 12 resides on a supporting substrate 14 as shown in FIG. 1. The mechanical and thermal properties of the supporting substrate 14 and the piezoelectric substrate 12 act in conjunction to render the SAW device 10 more stable to temperature variations. As illustrated in this example, a dual-mode SAW (DMS) device is provided, wherein at least two interdigitated transducers (IDT) 16 are placed between two interdigitated reflectors 18. Both the IDTs 16 and the reflectors 18 include a number of fingers 20 that are connected to opposing bus bars 22. For the reflectors 18, all of the fingers 20 connect to each bus bar 22. For the IDTs 16, alternating fingers 20 are connected to different bus bars 22, as depicted. Notably, the reflectors 18 and IDTs 16 generally have a much larger number of fingers 20 than depicted. The number of actual fingers 20 has been significantly reduced in the drawing figures in an effort to more clearly depict the overall concepts employed in available SAW devices 10 as well as the concepts provided by the present invention.
Notably, the fingers 20 are parallel to one another and aligned within an acoustic cavity, which essentially encompasses the area in which the reflectors 18 and the IDTs 16 reside. In this acoustic cavity, the standing wave or waves generated when the IDTs 16 are excited with electrical signals essentially reside within the acoustic cavity. As such, the acoustic wave energy essentially runs perpendicular across the various fingers 20. In the example illustrated in FIG. 1, one IDT 16 may act as an input while the other IDT 16 may act as an output for electrical signals. Notably, the IDTs 16 and the reflectors 18 are oriented in acoustic series, such that the acoustic wave energy moves along the cavity and perpendicularly across the respective fingers 20 of the IDTs 16 and the reflectors 18.
The operating frequency of the SAW device 10 is a function of the pitch (P). The pitch is the spacing between the interdigitated fingers 20 of the IDTs 16 and reflectors 18. An objective of most SAW designs is to maintain a consistent frequency response of the SAW device 10. If the spacing changes between the interdigitated fingers 20, the frequency response of the SAW device 10 changes. However, the spacing changes are only a part of the response change. Another factor that significantly affects the frequency response change in the SAW device 10 is the change in SAW velocity which occurs in response to the change in elastic properties of the piezoelectric substrate 12. Unfortunately, piezoelectric substrates 12 generally have a relatively high thermal coefficient of expansion (TCE) and a significant dependence on the temperature coefficient of velocity (TCV), and as temperature changes, the piezoelectric substrate 12 will expand and contract and the velocity will increase and decrease. Such expansion and contraction changes the pitch, or spacing, between the interdigitated fingers 20 as the velocity changes, with temperature variations, in an unfavorable way. Expansion and contraction of the piezoelectric substrate 12, along with an increase and decrease of SAW velocity changes the frequency response of the SAW device 10. The thermal coefficient of frequency (TCF=TCV−TCE) is a measure of how much the frequency response changes as a function of temperature. Given the need for a SAW device 10 having a frequency response that is relatively constant as temperature changes, there is a need for a piezoelectric substrate 12 that has an effective TCF that is relatively low. To obtain a low TCF, the piezoelectric substrate 12 needs to have a relatively low difference between the effective TCE and the effective TCV. This condition may coincide with simultaneously low TCE and TCV to limit expansion and contraction of the piezoelectric substrate 12 as temperature changes.
A piezoelectric substrate 12 having a higher TCE also injects issues during manufacturing of the SAW device 10. As noted, the piezoelectric substrate 12 is formed on a supporting substrate 14. The supporting substrate 14 generally has a significantly lower TCE than the piezoelectric substrate 12 and thus will not expand or contract as much as the piezoelectric substrate 12 as temperature changes. As such, the change in velocity is minimal for the supporting substrate 14 as temperature changes. As temperature changes during the manufacturing process, the piezoelectric substrate 12 tends to expand and contract more than the supporting substrate 14, which results in bending or warping of both the supporting substrate 14 and the piezoelectric substrate 12, as shown in FIGS. 2A and 2B. Bent and warped substrates lead to a litany of manufacturing issues during photolithography, dicing, mounting, packaging, and integration of the SAW devices 10 with other semiconductor components. Accordingly, there is a further need for a SAW device 10 with an effective TCE that is relatively low.